1. Field of the Invention
This invention relates to a lithium manganese composite oxide which can be used as a cathode active material and constitute a lithium secondary battery which is cheap, has large capacity and is excellent in charge-discharge cycle characteristic, a manufacturing method thereof, and a lithium secondary battery using the lithium manganese composite oxide as the cathode active material.
2. Related Background Art
With the miniaturization of the portable telephones and the personal computers, etc., the secondary battery with high energy density is required in the field of communication equipment and information relevant apparatus, so the lithium secondary battery has been widely used. Also, from aspect of resources problem and environmental problem, large request for the electric automobile exists in the field of the automobile, so the development of the lithium secondary battery which is cheap, has large capacity and is excellent in the cycle characteristic has been desired.
At present, for the secondary battery of 4V, the cathode active material of lithium secondary battery, LiCoO.sub.2 with the layered structure with rock salt type ordered cations (R3m) is adopted. In addition to easy synthesis and relatively easier handling, LiCoO.sub.2 is excellent in the charge-discharge cycle characteristic, so the secondary battery which uses LiCoO.sub.2 as the cathode active material has been mainly used.
However, since cobalt (Co) is few as the resources, the secondary battery which uses LiCoO.sub.2 as the cathode active material is difficult to deal with the future mass production of the large-capacity battery as the power source for electric automobile, and necessarily becomes expensive. In view of this, adoption of the lithium manganese composite oxide which contains the manganese rich as the resource and cheap as the constituent element instead of the cobalt as the cathode active material has been tried.
Among the lithium manganese composite oxide, one which has the spinel structure (Fd3m) and which is represented by composition formula of LiMn.sub.2 O.sub.4 is most stabilized. Although LiMn.sub.2 O.sub.4 with the spinel structure can constitutes a lithium secondary battery of 4V as the cathode active material, the theoretical discharge capacity of LiMn.sub.2 O.sub.4 is 148 mAh/g, which is smaller than 274 mAh/g of LiCoO.sub.2.
The lithium manganese composite oxide with the layered structure is expected next. LiMnO.sub.2 with the layered structure has the theoretical discharge capacity of 286 mAh/g, which is bigger than that of LiMn.sub.2 O.sub.4 with the spinel structure, so it can be used as the effective cathode active material.
As LiMnO.sub.2 with the layered structure, one which has the zigzag layered structure (Pmmn) of the rhombic system is well known. For example, Japanese Patent Laid-open No. 8-37027 shows to form LiMnO .sub.2 with the zigzag layered structure by firing the mixture of LiOH and Mn.sub.2 O.sub.3 mixed in the atomic ratio by Li/Mn=1/1. 05 in the vacuum at temperature of 600 to 800.degree. C. for 12 hours. However, this LiMnO.sub.2 with the zigzag layered structure has the problem that 30 cycles forms the limit for the charge-discharge cycle maintained rate at capacity of 200 mAh/g.
Among LiMnO.sub.2 with the layered structure, one which has a layered structure with rock salt type ordered cations (R3) which is same as LiCoO.sub.2 and LiNiO.sub.2 seems to be a good cathode active material for lithium secondary battery, but the synthetic method for it has not been established. On account of advance of the research afterwards, LiMnO.sub.2 with the layered rock salt type structure is synthesized by the method to be mentioned in the following.
(1) Japanese Patent Laid-open No.10-3921 shows the synthesizing method in which as the manganese source the inorganic salt such as MnO.sub.2, Mn.sub.2 O.sub.3, MnOOH or MnCO.sub.3, or the organic acid salt such as manganese acetate, manganese butyrate, manganese oxalate or manganese citrate is used; as a lithium source LiOH, LiNO.sub.3 or Li.sub.2 CO.sub.3 etc. is used; and water or the organic solvents such as alcohols is used, to synthesize LiMnO.sub.2 with the layered structure from them under saturated vapor pressure at the temperature of 100 to 300.degree. C.(2) J.Electrocem.Soc., and 1997, 144, 64 shows the method using KMnO.sub.4 as the starting material, to prepare LiMnO.sub.4 through the ion-exchange reaction, to hydrothermally treat it in the nitric acid acidity atmosphere at the temperature 160.degree. C. for three days and half, and to dehydrate it. (3) Nature 1996, 381,499 shows the synthesizing method to form NaMnO.sub.2 obtained by the reaction of MnO.sub.2 and NaOH at the temperature of 700.degree. C. in the argon atmosphere and to ion-exchange it with LiCl in the nonaqueous solvent.
However, in the secondary battery which uses LiMnO.sub.2 synthesized by the above method (1) as cathode active material, only the discharge capacity of 80 mAh/g can be obtained as is shown in Japanese Patent Laid-open No.10-3921, whereas the theoretical discharge capacity is 286 mAh/g. In the above method (2), in addition to the defect in the manufacturing process that it includes the multistage reactions and requires the accuracy in preparation of the solution by each stage of work, when the battery is constituted by using the LiMnO.sub.2 as the cathode active material, only discharge capacity of 50% or less of the initial capacity can be obtained after 10 cycles;, which deteriorates the charge-discharge cycle characteristic extremely. The above method (3) needs both of the synthesizing of NaMnO.sub.2 which is the precursor and the ion-exchanging reaction in the nonaqueous system, which leads to increase of the manufacturing cost. Also, it is reported LiMnO.sub.2 crystal synthesized thorough this process has the discharge capacity density of about 270 mAh/g as the cathode active material, but in fact, the charge-discharge cycle characteristic reduces down to 50% of the initial capacity after 50 cycles, thus involving problem in the stability of crystal structure.
It seems that this inferior cycle characteristic is essential problem because LiMnO.sub.2 used as the cathode active material has the layered rock salt type structure. In short, this problem is firstly resulted from that in case of the layered rock salt type structure, the lithium layer exists between the oxygen layers of high electronegativity, and the distance between the oxygen layers is enlarged by the electrostatic repulsive force therebetween as decrease of lithium amount due to charge of the battery, to make the crystal structure itself unstable. Above problem is secondary resulted from that as decrease of the lithium amount, LiMnO.sub.2 changes to LiMn.sub.2 O.sub.4 in composition, to transform to the more stable spinel structure.
The lithium secondary battery in which LiMnO.sub.2 with the layered rock salt type structure is adopted as the cathode active material can not be practical unless this problem of cycle characteristic is solved. Whereas, the effective technology which can solve the problem of this cycle characteristic has not existed conventionally.
In the beginning, the lithium secondary battery which lithium has started from the secondary battery which uses the metallic lithium for the negative electrode. However, because the dendrite is precipitated on the surface of metallic lithium in the negative electrode to cause the problem of short of the battery, so-called lithium ion secondary battery which uses the carbon material for the negative electrode has been used recently.
Whereas, when the carbon material is used as anode active material, the problem of retention in turn occurs. When the retention occurs, the part of the lithium intercalated into the negative electrode by the first charging is irreversibly incorporated into the negative electrode, and remaining in the negative electrode even after the succeeding discharge to thereby cause decrease of the discharge capacity of the battery. This retention is the peculiar problem to the carbon material and in somewhat unavoidable, as long as the carbon material is used as the anode active material.
As mentioned above, when producing of the cheap lithium ion secondary battery is intended, the lithium manganese composite oxide in which the cheap manganese constitutes is preferably used as the cathode active material. Although the theoretical discharge capacity of the above LiMnO.sub.2 with the layered rock salt type structure is large, the lithium ion secondary battery which adopts LiMn.sub.2 O.sub.4 with the spinel structure as the cathode active material is preferably used by the reason of stabilized crystal structure thereof.
However, the problem of retention occurred in the negative electrode becomes more serious in case of the lithium ion secondary battery which adopts LiMn.sub.2 O.sub.4 with the spinel structure as the cathode active material. In short, the amount of lithium returned to the positive electrode by the discharge drastically decreases, because the lithium amount irreversibly incorporated into the negative electrode using the carbon material is constant peculiar to the negative electrode, when LiMn.sub.2 O.sub.4 with spinel structure of small lithium contained amount per unit weight and of small theoretical discharge capacity is used as the cathode active material.
As the means for compensating for the retention which occurs in the negative electrode, Japanese Patent Laid-open No.8-162159 shows to intercalate, by using the metallic lithium, the lithium of the amount corresponding to the irreversible capacity in the negative electrode using carbon material by electrochemical manner in advance, before or after assembling the battery; while Japanese Patent Laid-open No.8-255633 shows to intercalate, by using the metallic lithium, the lithium of amount corresponding to the irreversible amount in the positive electrode at once, and discharging it after assembling the battery to intercalate it in the negative electrode.
However, in the above method, the metallic lithium active in the air is handled before assembling of the battery, and the complicated work which pays the careful attention in work environment is needed. Also, in this method, there is some possibility that the metallic lithium remains in the battery, which is not preferable for safety. In addition, the lithium intercalated in the negative electrode using carbon material is unstable, and is easily resolved into Li.sub.2 CO.sub.3.